(Spoiler: The RA Isn't Just Hype)
If you're deep into Subaru EJ257 engine builds — whether for street tuning, reliability mods, or just satisfying your inner machining nerd — you've probably heard whispers about the differences between standard STi pistons and those found in the “special” Type RA models.
I did a precise, no-BS side-by-side teardown using real tools: Mitutoyo micrometer, dial caliper, and an Acculab scale, all at a controlled 68°F ambient temp. Both pistons are OEM “B” bore units, but the STi versions come from Karl Schmidt Unisia, while the RA pistons are made by Hitachi.
Here are the key measurements and differences I documented:
- **Major skirt diameter** — RA: 3.9170″ | STi: 3.9172″ (STi is microscopically larger)
- **Top & second ring gaps** — RA: 0.012″ & 0.019″ | STi: 0.010″ & 0.017″ (STi runs tighter gaps)
- **Major skirt width** — RA: 2.490″ | STi: 2.460″ (RA is wider)
- **Top & second ringland thickness** — RA: 0.180″ & 0.160″ | STi: 0.180″ & 0.150″ (RA's second ringland is thicker)
- **Pin boss strut area thickness** — RA: 0.365–0.400″ | STi: 0.200–0.320″ (RA is noticeably beefier here)
- **Skirt thickness (thinnest point)** — Both: 0.065″ (identical)
- **Piston weight** — RA: 424–425 grams | STi: 414–415 grams (RA is about 10g heavier)
- **Wrist pins** — Identical: 0.220″ wall thickness, same length and weight
- **Dish volume** — Both ~22–23cc, but the RA has a slightly deeper dish with a steeper center point (which could influence burn characteristics — though without an RA-specific timing map, it's hard to say exactly how much)
The RA piston is almost certainly still hypereutectic aluminum (like most OEM Subaru pistons), not forged. That extra 10 grams didn't go to waste — it seems to have been strategically added to reinforce critical areas: thicker pin boss struts, a wider skirt, and what "feels" like a slightly thicker crown. Interestingly, many aftermarket forged EJ25 pistons drop the top ringland down closer to 2mm from the crown to escape extreme heat zones, but the RA keeps it higher up, suggesting better heat management in a factory setup.
The RA piston isn't designed to be the go-to for ultra-high-horsepower drag monsters or "Mexico Racing League" builds (you know the type — track-only, church-on-Sunday reliability not required). But for street cars, moderate boost levels, or builds aiming for solid longevity (think 150,000+ miles with occasional spirited driving), it looks like a seriously underrated factory option. If you're willing to blueprint & open the ring gaps up and match them properly, the added material in the right spots could make them punch above their weight compared to standard STi pistons.
What do you think — ever run RA pistons in a build? Or sticking with forged aftermarket for peace of mind? Drop your thoughts in the contact box.